For Aquinas, the rational appetitive function was the ground
of moral behavior. In his fascinating book, Descartes' Error,
Antonio Damasio reports a famous case of brain damage caused by
a metal rod driven through the skull of a railway employee, named
Phineas Gage. Gage recovered almost entirely from his physical
disabilities, except for loss of sight in one eye. It is surprising,
of course, that Gage survived such a traumatic event at all, but
more surprising is the fact that his personality was completely
changed as a result of the accident. Gage's doctor describes how
"the equilibrium or balance, so to speak, between his intellectual
faculty and animal propensities" had been destroyed. The
changes became apparent as soon as the acute phase of brain injury
subsided. He was now "fitful, irreverent, indulging at times
in the grossest profanity which was not previously his custom,
manifesting but little deference for his fellows, impatient of
restraint or advice when it conflicts with his desires, at times
perniciously obstinate, yet capricious and vacillating, devising
many plans of future operation, which are no sooner arranged than
they are abandoned. . . ." .

These new personality traits contrasted sharply with the "temperate
habits" and "considerable energy of character"
Phineas Gage was known to have possessed before the accident.
Previously, he had "a well balanced mind and was looked upon
by those who knew him as a shrewd, small businessman, very energetic
and persistent in executing all his plans of action." So
radical was the change in him that friends and acquaintances could
hardly recognize the man. They noted sadly that "Gage was
no longer Gage." So different a man was he that his employers
had to let him go shortly after he returned to work. The problem
was not lack of physical ability or skill; it was his new character.

Damasio uses this story to introduce his research on brain
localization. That is, by a careful analysis of Gage's skull,
Damasio has been able to determine exactly which parts of the
brain were destroyed by the iron rod. He infers from this and
other similar cases that specific regions are essential to the
sort of practical reasoning Gage became incapable of performing.
To return to Aquinas's language, Gage lost his ability to be attracted
to the Good.